1. Field of the Invention
The present invention relates generally to couplings between laser sources and multimode optical fibers, particularly for purposes of supporting consistent bandwidths over short distances.
2. Technical Background
Local area optical networks within and between buildings often employ multimode fibers for conveying high speed signals. Cores of the multimode fibers are larger in diameter than cores of single mode fibers and can carry more power. The larger core diameters also simplify splicing and alignment of the fibers with each other or with other network connections.
Most multimode fibers are tuned for use with conventional light emitting diode (LED) sources. These sources have limited modulation capability and, therefore, can only support data rates up to approximately 500 MB/s. In order to support higher speeds, such as 1.25 GB/s (as in the Gigabit Ethernet standard), systems require laser sources. However, lasers such as Fabry Perot (FP) lasers and vertical cavity surface emitting lasers (VCSELs), which are preferred for their higher power, narrower spectral width, higher modulation rates, and ability to couple to multiple arrays, generally couple less reliably to multimode fibers.
Compared with conventional LED sources, the lasers (FP lasers and VCSELs) produce smaller spot sizes that underfill the multimode fibers (i.e., not all of the modes are equally excited). Since each of the modes conveys light a little differently, the bandwidth varies between the different fill conditions. The smaller spot sizes also concentrate energy near the centers of the multimode fibers where refractive index profiles are particularly difficult to control. Small changes in the alignment of the lasers with the multimode fibers can produce further variations in the bandwidth.
Although the multimode fibers can be adjusted with respect to the laser sources to optimize bandwidth, the adjustments require in situ testing or other special attentions that can complicate and add to the costs of network installations. For example, repeated trials can be required to identify positions of alignment that support adequate bandwidth and special devices must be added to the link to ensure alignment is maintained.